1. Field of the Disclosure
The present disclosure relates to a control circuit, a power conditioner including the control circuit, and a photovoltaic system.
2. Background Information
A photovoltaic system generally converts a direct-current (DC) power from a solar cell to an alternating-current (AC) power of a commercial frequency that is interconnected to a system by a power conditioner and also supplies the converted AC power to an in-house load connected to a commercial power system, where excessive power is reversely flowed to the system side if the AC power is greater than the consumption power of the in-house load. In such a power conditioner, a non-insulating type excelling in power conversion efficiency is greatly used (see e.g., Japanese Unexamined Patent Publication No. 2002-10496).
FIG. 14 shows a configuration example of a photovoltaic system including a non-insulating type power conditioner. A power conditioner 100 operates while being interconnected to a commercial power supply 2. The power conditioner 100 includes a smoothing capacitor 101 for smoothing a generated power output from a solar cell panel 1, an inverter 102 of PWM control, a filter 103 including a reactor, and a control circuit (not shown). In the power conditioner 100, the generated power output from the solar cell panel 1 is smoothened by the smoothing capacitor 101. The inverter 102 is configured by switch elements 104 to 107 including four MOSFETs connected in antiparallel by a diode. In the power conditioner 100, the switching control of turning ON/OFF the switch elements 104 to 107 in the inverter 102 at a high frequency of around 18 kHz is performed to convert the generated power output of the solar cell panel 1 smoothened by the smoothening capacitor 101 to the AC power synchronized with the commercial power system for output. The power conditioner 100 supplies the thus converted AC power to a load (not shown) through the filter 103, or reversely flows the same to the system side.
The mainstream of the solar cell configuring the solar cell panel 1 is a crystal system solar cell excelling in conversion efficiency. Meanwhile, an inexpensive thin film solar cell has been used in which the usage amount of silicon, which is a raw material, can be greatly reduced, a production process is simple, and an area can be increased. The thin film solar cell made of amorphous silicon is known to degrade over the years when the negative electrode side of the solar cell becomes lower than the ground potential.
The negative electrode side of the thin film solar cell needs to be the ground potential in order to prevent degradation in the thin film solar cell. However, since the level of reference potential differs for the DC side and for the AC side in the non-insulating type power conditioner 100, the negative electrode side of the solar cell, which is the input side of the power conditioner 100, cannot be the ground potential.
The present applicant thus already proposed a non-insulating type power conditioner capable of preventing degradation of the thin film solar cell and a photovoltaic system using the same (Japanese Patent Application No. 2009-61916 filed Mar. 13, 2009).